Laser welding is a metal joining process in which a laser beam is directed at a metal workpiece stack-up to provide a concentrated heat source capable of effectuating a weld joint between the component metal workpieces. In general, two or more metal workpieces are first aligned and stacked relative to one another such that their faying surfaces overlap and confront at an intended welding site. A laser beam is then directed at a top surface of the workpiece stack-up. The heat generated from the absorption of energy from the laser beam initiates melting of the metal workpieces and establishes a molten weld pool within the workpiece stack-up. The molten weld pool penetrates through the metal workpiece impinged upon by the laser beam and into the underlying metal workpiece or workpieces. When the laser beam has a high enough power density, a keyhole is created within the molten weld pool directly underneath the laser beam (a process known as “keyhole welding”). A keyhole is a column of vaporized metal derived from the metal workpieces within the workpiece stack-up that may include plasma.
The keyhole provides a conduit for energy absorption deeper into workpiece stack-up which, in turn, facilitates deeper penetration of the molten weld pool and a narrower weld pool profile. As such, the keyhole is normally controlled to penetrate into the workpiece stack-up across each faying interface, but only partially through the bottommost metal workpiece. The keyhole is typically created in very short order—typically miliseconds—once the laser beam impinges the top surface of the workpiece stack-up. After the keyhole is formed and stable, the laser beam is moved a short distance along a weld path. Such movement of the laser beam leaves behind molten workpiece material in the wake of the corresponding travel path of the keyhole and molten weld pool. This penetrating molten workpiece material cools and solidifies in the same direction as the forward movement of the laser beam to provide a trail of re-solidified workpiece material that fusion welds the workpieces together.
The automotive industry frequently uses remote laser welding to join metal sub-assemblies into finished parts that can be installed on a vehicle. In one example, a vehicle door body may be fabricated from an inner door panel and an outer door panel that are joined together by a plurality of laser welds. The inner and outer door panels are first stacked relative to each other and typically secured in place by clamps. A moveable optic laser head then intermittently directs a laser beam at multiple weld sites around the stacked panels in accordance with a programmed sequence to form the plurality of laser welds. At each weld site where a laser weld is to be formed, the laser beam is directed at the stacked panels and conveyed along a predefined weld path, which may be configured to produce a discrete spot weld or a continuous seam weld. The process of laser welding inner and outer door panels (as well as other vehicle part components such as those used to fabricate hoods, deck lids, etc.) is typically an automated process that can be carried out quickly and efficiently.
The use of remote laser welding to join coated metal workpieces together can present challenges. For example, zinc-coated steel workpieces include a thin outer coating of zinc for corrosion protection. Zinc has a boiling point of about 906° C., while the melting point of the base steel substrate it coats is typically greater than 1300° C. Thus, when zinc-coated steel workpieces are laser welded together, high-pressure zinc vapor is readily produced at the surfaces of the steel workpieces. The zinc vapor produced at the faying surfaces of the stacked steel workpieces is forced to diffuse into and through the molten weld pool produced by the laser beam unless an alternative escape outlet is provided through the workpiece stack-up. When an adequate escape outlet is not provided, zinc vapors may remain trapped in the molten weld pool as it cools and solidifies, which may lead to defects in the resulting weld joint—such as spatter and porosity—that degrade the mechanical properties of the joint to such an extent that the joint may be deemed non-confirming. The vaporization of zinc coatings on steel workpiece surfaces during laser welding has the tendency to be most disruptive when the faying surfaces of the steel workpieces are tightly fit with a zero-gap interface therebetween.
To deter zinc vapor from diffusing into the molten weld pool, and ultimately causing weld defects to be present in the re-solidified workpiece material of a weld joint formed between steel workpieces (at least one of which is zinc-coated), the workpieces are oftentimes scored with a laser beam before laser welding takes place to create spaced apart protruding features on one or more of the faying surfaces of the steel workpieces. The protruding features impose a gap of about 0.1-0.2 millimeters between the faying surfaces of the steel workpieces, which provides an escape path to guide zinc vapors away from the weld site during the laser welding process. But the formation of these protruding features adds an additional step to the overall remote laser welding process and tends to produce undercut weld joints that, while acceptable, are not as desirable as weld joints that are formed between steel workpieces that do not have an intentionally imposed gap formed between their faying surfaces to facilitate vapor escape.